Add Observable and SynchronizedObserver utilities

[Copilot]

Ports Observable and SynchronizedObserver utility types from CliWrap into PowerKit.

New types

• Observable<T> — IObservable<T> implementation backed by a subscribe delegate.
• Observable (static) — factory methods:
  • Create<T> — wraps a subscribe delegate into an observable.
  • CreateSynchronized<T> — same, but wraps the observer in SynchronizedObserver<T> before invoking the delegate, ensuring thread-safe delivery.
• SynchronizedObserver<T> — IObserver<T> decorator that serializes OnNext/OnError/OnCompleted calls via lock.

Both files are guarded with #if NET40_OR_GREATER || NETSTANDARD || NET since IObservable<T>/IObserver<T> don't exist in .NET 3.5.

Usage

var observable = Observable.CreateSynchronized<string>(observer =>
{
    // observer calls are automatically serialized across threads
    return Disposable.Create(() => { /* cleanup */ });
});

[Copilot]

Pull request overview

Ports lightweight IObservable<T>/IObserver<T> utilities into PowerKit (aligned with the project’s existing “small internal building blocks” approach) and adds tests to validate subscription behavior and synchronized delivery across threads.

Changes:

• Added Observable<T> + Observable factory helpers to create observables from a subscribe delegate.
• Added SynchronizedObserver<T> to serialize observer callbacks via lock, and Observable.CreateSynchronized<T> to apply it automatically.
• Added unit tests covering basic observable behavior and a multi-threaded delivery scenario.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
PowerKit/SynchronizedObserver.cs	Adds an IObserver<T> decorator that serializes OnNext/OnError/OnCompleted calls.
PowerKit/Observable.cs	Adds a minimal IObservable<T> implementation + factory helpers including synchronized creation.
PowerKit.Tests/ObservableTests.cs	Adds coverage for Observable.Create, disposal propagation, and synchronized/thread-safe delivery.

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[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.35%. Comparing base (e1aaff9) to head (3473b18).
⚠️ Report is 4 commits behind head on prime.

Files with missing lines	Patch %	Lines
PowerKit/SynchronizedObserver.cs	71.42%	4 Missing ⚠️
PowerKit.Tests/ObservableTests.cs	97.91%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            prime      #46      +/-   ##
==========================================
- Coverage   94.36%   94.35%   -0.02%     
==========================================
  Files          82       87       +5     
  Lines        1740     1878     +138     
  Branches      141      152      +11     
==========================================
+ Hits         1642     1772     +130     
- Misses         72       77       +5     
- Partials       26       29       +3     

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